Since he openraises very big I think I would just call even though I'm oop. Or if you don't feel comfortable playing that hand you should just fold - 3betting is quite risky taking into consideration his position and strange bet amount. As played it's pretty much standard postflop.

AJ is very bad hand to play against a raise. If you do want to play this one, I would call preflop.
On flop I would just Cbet/fold. You get quite good odds, but you may be down to 2 outs (backdoor FD).

Against an unknown open raising for 6bb from MP i'd just muck it out of position, i don't think it's worth it until i know more about his weird betsizing and playing tendencies. For all you know he decided that AA was worth a lot of money and you're dead already. The fact that it's uSNL is more an argument for playing ABC poker than for trying to expand your value 3betting range against an unknown making weird moves.

I don't like calling either until i know his range here is wide enough for me not to run into dominating hands too often. Out of position you're gonna have to let him bet into you on Axy and Jxy flops or be confident that he will call your donkbets, which he might not do unless he has a piece of the flop, which will sometimes be bigger than yours since your kicker isn't that great.